Switch to nodenext module

[perrin4869]

Merging this will ensure we maintain compatibility with nodenext modules (for example, we don't forget to add .js extensions).
Unfortunately, this is currently blocked by auth0/jwt-decode#130, and maybe even by ts-node, so keeping it as a draft

[coveralls]

Coverage remained the same at 98.904% when pulling e780b4c on feature/nodenext into 7f76aaf on master.

[perrin4869]

ok I managed to find a way to override the official typings temporarily at least
We can remove those if/when auth0/jwt-decode#130 is ever merged

[phieronymus]

Thanks!